The production of photopolymer reliefs for flexographic printing typically involves imagewise exposing the photosensitive layer of an element with ultraviolet radiation. Unexposed areas are washed off using a suitable solvent. Solvent is removed by evaporation and, if necessary, the surface is treated to remove tack.
Generally, aqueous alkaline solutions or organic solvents such as saturated hydrocarbons, aromatic hydrocarbons, aliphatic ketones, and especially chlorinated hydrocarbons such as trichloroethylene, tetrachloroethylene, or trichloroethane either by itself or admixed with low molecular weight alcohols constitute suitable developer solvents depending upon the binder used to formulate the photosensitive layer.
Known developer solvents exhibit a number of undesirable properties. Frequently, extended wash-off times are needed which leads to swelling of the imagewise exposed photosensitive layer and to dissolution of the crosslinked areas due to the lengthy development time. Lengthy drying times are usually needed to completely remove the developer solvent. Otherwise, a defective printing surface results because it is too soft. Another disadvantage to using some solvents is their high flammability which requires that the plant be explosion proof. It is very expensive to ensure that a plant is explosion-proof. Other solvents such as chlorinated hydrocarbons are extremely toxic and present serious safety and disposal problems.
Journal of the Electrochemical Society, pages 1628-1630, Vol. 126, No. 9 (Sep. 1979), and Journal of Vacuum Science and Technology, pages 1121-1126, Vol. 19, No. 4 (Nov./Dec. 1981) cited in PCT Application No. WO 86/00725 which was published on Jan. 30, 1986, describe photoresists containing chloromethylated polystyrene which are developed by dipping the resists in acetic acid-n-pentyl ester and subsequently treating with isopropyl alcohol.
German Preliminary Published Application No. 3,315,118 describes mixtures of good solvents such as acetic acid alkyl esters having one to five carbon atoms and poor solvents such as ethylene glycol alkyl ethers as developer solvents for radiation-sensitive negative protective coatings made of an aromatic vinyl polymer.
The low molecular weight acetic acid esters described above are unsuitable developer solvents because they pose health, safety and environmental problems. Furthermore, it is known that low molecular weight acetic acid esters cause printing reliefs to swell.
European Patent Application Publication No. 221,428 which was published on May 13, 1987 describes mixtures of propylene glycol alkyl ethers and propylene glycol ether acetates, preferably in a 1:1 ratio, as coating removal agents for photoresists, particularly for those resists containing novolak resins. No differentiation between exposed and unexposed areas is needed because the entire hardened resist composition is removed. Swelling also does not present a problem as no resist material remains.
A developing process for photosensitive coating is described in German Patent 2,459,156 which was published on Nov. 6, 1975. The photopolymers are degraded by exposure to ultraviolet radiation and the low molecular weight photodegradation products are dissolved out in the developing step. Development is carried out using alkyl acetates or ketones having seven to nine carbon atoms at an elevated temperature. At room temperature, the solvents used, e.g., isoamyl acetate, exhibit poor dissolution properties. n-Hexyl acetate is also generally designated as a poor solvent.
Accordingly, there is a need for a process to produce flexographic printing reliefs using developer solvents which remove the non-crosslinked portions of the imagewise exposed layers rapidly with good edge sharpness and without causing the photosensitive layer to swell or removal of the exposed areas. These reliefs must be dried quickly and completely resulting in a stable, uniform printing surface. Furthermore, the wash out system must be environmentally safe and not pose any disposal problems. The system should be easy to handle and monitor.